Polarographic method and apparatus



United States Patent Ofiice Patented Jan. 15, 1953 3,973,757 POLAROGRAPHIC METHOD AND AEPARATUE) Rolf K. Ladisch, 255 Windermere, Lansdowne, Pa. Filed June 22, 1959, Ser. No. 821,978 16 Claims. (Cl. 204-1) The present invention relates to an electrochemical method and apparatus in the field of polarographic analysis. Characteristics, as well as limitations and potentialities of polarography, derivative polarography, differential polarography, oscilloscopic polarography, controlled potential separations, and amperometry have been extensively discussed in the literature. See for instance Chemische Analysen mit dem Polarographen, by H. Hohn, J. Springer Verlag, Berlin 1937; Polarography, by I. M. Kolthofr" and J. J. Lingane, lnterscience Publishers, New York, 1952; Die Polarographie in der Medizin, Biochemie und Pharmazie, by M. Brezina and P. Zurnan, Ak-ad. Verlagsges, Leipzig, 1956.

These methods are based on the fact that electrolysis of a solution between a polarizable electrode and a nonpolarizable electrode produces current-voltage relationships that are typical for the quantity and quality of the investigated test species in the solution. The outstanding feature of the classic polarographic electrolysis cell is the polarizable dropping mercury electrode, i.e., mercury droplets being discharged periodically into the solution from a fine bore capillary under a driving head of mercury. This very feature, which has permitted the initiation of extremely useful polarographic methods in research work, seems now to militate against :a more general use of polarography as a common analytical method and particularly as a tool for monitoring and controlling industrial process streams. The periodic growth and fall of the mercury droplets cause oscillations in the current-voltage curves which are an obstacle in evaluating such curves. Also an undesired condenser current builds up whenever a new mercury droplet is being formed at the capillary. Furthermore, the size of the droplets is necessarily small, thus limiting the method to a fraction of the sensitivity that could otherwise be obtained. In addition, formation of these tiny droplets is a delicate process and is therefore affected by a number of incidental factors, such as mechanical vibrations, slant of capillary, pulsation of test solution into the mouth of the capillary between drops, etc. it should be remembered in this connection that the reproducibility of the droplets with regard to their drop time and mass of mercury per drop must be practically perfect at all times to permit proper evaluation of the polarogram.

Many attempts have been made to substitute better means for the dropping mercury electrode, such as multicapillaries, ejection of a stream of mercury from the capillary into the test solution, bent electrodes to increase drop size, forced dislodgment of droplets from the capillary in rapid succession, synchronization of mercury droplets being discharged from pairs of capillaries, oscilloscopic sweeps over successive drops at definite recurring time intervals, etc. Some of these methods are cumbersome to practice, some are extremely complicated if not impracticable, and most of them have not produced more reliable data than the original polarographic method, although in cases of special application, some benefit has been derived from these suggestions.

The present invention employs simple polarizable mercury electrodes in lieu of the classic dropping mercury electrode. Essentially, the electrodes of this invention consist of a small amount of mercury that is separated from but electrolytically is put in communication with the test solution by means of a thin porous wall of small dimensions. Preferably the mercury is held against the porous wall under a slight pressure. In this arrangement,

. 2 the test solution under investigation migrates through the porous wall to be electrodeposited on the mercury. The solid matrix of the porous wall must be chemically inert to both the mercury and the test solution under investigation. It must not conduct electric current. The structure of said wall should be as open as possible, i.e., the pore volume should be as great as is consistent with a sufiicient structural rigidity to build and operate such an electrode. The pores in said wall should be small enough to prevent the mercury from entering them, but should be large enough to permit the test solution to diffuse freely through said porous Wall to the mercury. In general, porous media with controlled porosities which are available in the trade, such as microporous ceramic material with a pore volume of about 40% and an average pore diameter of about 5 to 10 microns, or fritted sintered glass with a similar degree of porosity and a similar pore size, are preferred in connection with pressures on the mercury ranging from about mm. Hg to 200 mm. Hg. The porous wall should have a thin cross section to facilitate the migration of the test solution to and from the mercury; it should be just thick enough to have suflicient structural strength for constructing and handling the electrode. The above mentioned microporous ceramic media may be formed into objects of the desired shapes with wall thicknesses of the order of onesixteenth of an inch or even less.

The mercury in this electrode is preferably held against the porous wall under a slight pressure either derived from an elevated supply of mercury connected to it, or from a pump driving the mercury through the electrode along the porous wall, or by any other suitable means.

pressure aids greatly in maintaining a coherent mercury surface at the boundary between the mercury and the porous wall, and thus it substantially eliminates migration of the test solution beyond the porous wall into that portion of the electrode which is assigned to the mercury.

The mercury may be kept stationary within the electrode, provided it is frequently renewed and provided the duration of test runs is limited to short periods of time, so as not to contaminate it appreciably with deposits of the electrode reaction. However, flow or streaming of the mercury through the electrode along the porous wall is preferred, particularly in apparatus designed for continuous analysis, such as for process streams of the chemical industry. The flow of. the mercury facilitates a continuous renewal of the mercury surface in the region where the electrode process takes place.

The test solution contacting the mercury in the electrode via the porous wall may be stationary since the rate of migration of the test species to the mercury is generally greater than the depletion of the test species in the solution. The depletion is generally minimal, because very low current densities of the order of a few microamperes are normally used to deposit the test species on the mercury. However, agitation and/ or flow of the test solution along the porous wall does not interfere with the analysis as would be the case with most polarographic cells. employed heretofore. This feature is particularly useful in equipment facilitating the continuous analysis of an industrial process stream or the like.

The polarizable electrode of the present invention offers a number of advantages over conventional dropping mercury electrodes, among which are simplicity of design and operation, increased sensitivity, increased resolution of waves belonging to substances with a similar decomposition potential, substantial elimination of the condenser current, greatly increased readability of polarograms due to elimination of oscillations in the curves, minimization of'polarographic maXima, and in general ruggedness, since the electrode of theinvention is neither sensitive to mechanical vibrations, nor to agitation of the test solution during analysis, nor to small changes in the height of the driving head of mercury.

In the accompanying drawings two embodiments of the invention are shown by way of illustration. In a companion application filed September 4, 1959, Serial No. 838,131, I have shown two additional polarographic cells which are within the scope of the broad claims applied hereto.

In said drawings:

FIG. 1 is a somewhat diagrammatic vertical section through a polarographic cell constructed in accordance with the invention, shown full size; and

FIG. 2 is a similar vertical section through another polarographic cell, also constructed in accordance with the invention, this form being particularly suited for the continuous polarographic analysis such as for process streams in the chemical industry.

Referring first to FIG. 1, there is shown a mercury reservoir 11 having a stand tube 12 leading on from the bottom of the reservoir. The mercury reservoir 11 may be made from glass or epoxy resin or other suitable material, and contains a supply of mercury 13. Stand tube 12 is connected at its lower end to a short length of fine bore capillary tubing 14 by means of a rubber sleeve 15. The capillary tubing 14 preferably has a length of about 30 mm., an outside diameter of approximately mm., and a bore of the order of 90 microns diameter. To the lower end of capillary tubing 14 is cemented a. ceramic microporous tubing 16 having an outside diameter of approximately 5 mm., a length of about 5 mm., and a bore of 1 to 2 mm. diameter. Epoxy resin cement may be used for the cementing. The average pore size of microporous tubing 16 is about 5 to 8 microns, but smaller or larger pore sizes may be employed successfully. Microporous tubing 16 is cemented at its lower end to capillary tubing 17 having quite similar dimensions to those of capillary tubing 14. Tubing 14 extends partly into an open container or beaker 18, and microporous tubing 16 and capillary tubing 17 are entirely surrounded by said container. A layer or pool of mercury 19 is in the bottom of container 18, being a conventional non-polarizable mercury pool electrode. It will be understood that other types of non-polarizable electrodes well known in the art may be employed instead. The distance between the mercury level in reservoir 11 and the lower end of capillary 17 may be of the order of 150 mm. Test solution 29 is poured on top of mercury pool 19 and fills container 18 to a level somewhat above the upper end of microporous tubing 16. The porous wall of tubing 16 is such as to permit the test solution to migrate freely but does not permit the mercury to enter its pores. Mercury 13 in reservoir 11 and mercury 19 in container 18 are connected by means of electrical conductors C C to a source of electricity and other apparatus adapted for registering polarographic data, in the manner well known to those skilled in the art. In this connection, reference may be made to FIG. 5 of applicants Patent No. 2,708,657, dated May 17, 1955, and pertinent portions of the specification of said patent.

In operation, mercury 13 flows slowly from reservoir 11 through stand tube 12, capillary 14, microporous tubing 16, and capillary 17 to be discharged from the lower end of capillary 17 in the form of droplets D. Electro-deposition of the investigated test substance on mercury flowing from reservoir 11 occurs mainly within the confines of microporous tubing 16, with an additional small deposition on the mercury droplets D. The polarographic current derived from the interaction between the test solution 20 and mercury droplets D amounts generally to less than 10% of the overall current extracted from this cell, which is satisfactory for most practical purposes in minimizing oscillations in the polarogram. Other advantages of this cell over conventional apparatus are elimination of the condenser current, increase in overall sensitivity, and increase in resolution with respect to test substances having similar decomposition potentials.

It will be obvious to those skilled in the art that the small current caused by the interaction of the test solution 20 with mercury droplets D may be eliminated, if desired, for example by collecting the mercury droplets in a small container (not shown) shielded from the test solution 20.

Referring now to the embodiment shown in FIG. 2, a polarographic cell is shown with a mercury reservoir 22, stand tube 23, and mercury supply 24. As illus trated, the mercury reservoir 22 and the stand tube 23 may be a unitary body made from epoxy resin (or any other suitable material) which has a cup-like upper end and is machined at its lower end to form a circular chamber 25. The stand tube 23 is a central bore leading 011 from the bottom of the reservoir 22. Inside chamber 25 and around the projecting neck 23, which continues bore 23, is wound an anode wire 26 of an appropriate metal, such as silver. The characteristics of anode wires used in polarography need not be explained, since they are well known to those skilled in the art. Anode wire 26 leads to a terminal 27 via a Teflon bushing 28. This arrangement facilitates easy interchange of the anode wire 26 for the purpose of cleaning or replacement. Body 29, likewise machined from epoxy resin or any other suitable material, closes the described polarographic cell at the bottom. Sealing is efiected by means of rubber O-ring 30 and three screws 31, the latter being positioned equiangularly around the circumference of body 29. For convenience of illustration, only one screw 31 is shown in the drawing. Body 29 has an inner cylindrical space 32, as shown, which forms an extension of chamber 25 after body 29 and reservoir 22 have been screwed together. A small bore 33 is drilled along the central axis of body 29 extending into a counterbore 34. The counterbore 34 has a co-axial groove 35 for O-ring 35 to receive and form a seal around capillary 36. Capillary 36 may have a length of about 50 mm. and has a bore of the order of to microns.

A piece of microporous ceramic tubing 37 is cemented to the inside of body 29 by means of epoxy cement or other suitable cement, to be co-axial with bore 33. The microporous tubing 37 may have a length of about 5 mm., a diameter of about 5 mm., and a bore of approximately 1 to 2 mm. Washer 38, consisting of rubber or tetrafluorethylene resin or any other suitable material, makes a tight seal between stand tube extension 23 and microporous ceramic tubing 37 after the cell has been assembled. Body 29 has a nipple 39 for the introduction of test solution (not shown) into chamber 32 of the cell. A nipple 40 leading off from the upper part of chamber 25 is provided for the removal of the test solution from the cell. The nipples 39, 40 may be made from epoxy resin and they may be cemented into their respective positions illustrated in the drawing. Mercury 24 in reservoir 22 and terminal 27 are connected by means of electrical conductors (not shown) to apparatus suitable for the determination of polarographic current-voltage data in the manner well known in the art.

In operation, mercury 24 flows down in stand tube 23, through microporous ceramic tubing 37, where electro-deposition of the investigated test species on the mercury takes place, then through bore 33 and capillary 36 to be discharged from capillary 36 in the form of droplets D.

Capillary 36, which is frictionally held in body 29 by means of O-ring 35", may be manually pulled down in bore 34 for a substantial distance and it may be pushed back again to the position shown in the drawing, without losing the sealing action of O-riug 35 on the capillary 36. This pumping action facilitates the 5, removal of air which may be entrapped in stand tube 23 and microporous tubing 37 when first filling the as sembly with mercury.

When using either the formof FIG. 1 or the form of FIG. 2, essentially the same method is followed, viz., putting a mass of mercury under slight pressure into contact with one face of a relatively thin porous chemically inert wall whose pores are small enough to preclude the mercury from entering them, and putting a test solu tion under investigation into contact with the other face of said porous wall and permitting the test solution to migrate through said wall to be electro-deposited on the mercury, and taking current-voltage readings during said electro-deposition. The porous wall need not be provided by a tube but can be a part of a rod, as disclosed in the companion application Serial No. 838,131. The invention may be embodied in several other forms of apparatus neither shown nor described.

What I claim is:

1. Electrochemical apparatus comprising, in combination, a source of mercury; means to conduct the mercury away from said source; a microporous body supported, constructed and arranged so that the mercury-conducting means discharges mercury into it; a container surrounding said microporous body and adapted to hold part of the test solution under investigation; the pores of said microporous body being so small that the mercury in said body cannot enter said pores but the test solution may migrate freely through saidpores to make contact with the mercury in said body; means supported, constructed and arranged to receive the mercury flowing through said microporous body and adapted to discharge the mercury at a slow rate, a suitable electrode positioned for electrolytic contact with the test solution; and means to apply an electrical potential to the mercury versus said electrode to produce a desired electrode reaction between the test solution and the mercury in said body.

2. The invention defined in claim 1, wherein the microporous body is of ceramic material which is chemically inert to mercury and to the test solution and which does not conduct electricity, the pore volume of said microporous body being approximately 40%.

3. Electrochemical apparatus comprising, in combination, a source of mercury; a container below the source of mercury; a non-polarizable mercury pool electrode in said container; said container being adapted to hold part of the test solution under investigation lying on top of the mercury pool electrode; means to conduct mercury away from said source of mercury; a capillary tube adapted to receive mercury from said mercury-conducting means; a microporous tubing adapted to receive mercury from the discharge end of said capillary tube; a second generally vertical capillary tube adapted to receive mercury from said microporous tubing and adapted to discharge mercury in droplets from its lower end, which is within said container and spaced above said mercury pool electrode; the microporous tubing being also in the container and being adapted to be immersed in any test solution that may be put in the container; the size of the pores of the microporous tubing being such that the mercury contained therein cannot enter said pores but the test solution in the surroundin container may migrate freely to the mercury within the microporous tubing; and means to apply an electrical potential to the mercury versus said mercury pool electrode to produce a desired electrode reaction between the test solution and the mercury in said microporous tubing.

4. The invention defined in claim 3, wherein the microporous tubing is a ceramic tube which does not conduct electricity, which is inert to mercury and to the test solution, and which has an outside diameter of approximately mm., a length of about 5 mm., and a bore of 1 to 2 mm. diameter, with pores averaging 5 to 3 microns.

5. Electrochemical apparatus comprising, in combination, a source of mercury; means to conduct mercury by gravity away from said source; a walled chamber below the source of mercury; inlet and outlet means provided on the walls of said chamber to permit the test solution under investigation to flow through said chamber; a microporous tubing within said chamber and connected to the discharge end of said mercury-conducting means so as to receive and conduct mercury; a capillary tube mounted so as to receive and conduct mercury from the microporous tubing and from mercury droplets at its lower end; an anode wire mounted within said chamber and extending outwardly through the walls thereof; and means to apply an electrical potential to said mercury versus said anode wire so as to produce a desired electrode reaction between the test solution and the mercury in said microporous tubing.

6. The invention defined in claim 5, wherein there is a resinous plastic body having a cup-like upper end in which the source of mercury is contained; said walled chamber being formed partly within the lower end of said resinous plastic body; said mercury-conducting means being a bore in said body which continues through a neck projecting from and attached to the lower end of said body and lying within said chamber; said anode wire being wound on the outside walls of said neck, and the microporous tubing being secured to and sealed upon the lower end of said neck.

7. The invention defined in claim 5, wherein said walled chamber is formed partly on the lower end of a resinous plastic body, another resinous plastic body being removably secured to and sealed in said walled chamber to close and seal the lower end of the same; the microporous tubing being interposed between the two plastic bodies and having a central bore leading from said mercury source to the microporous tubing, and the lower plastic body having a bore leading from said microporous tubing to said capillary tube.

8. The invention defined in claim 7, wherein the lower resinous plastic body has a bore of substantial length in which said capillary tube has a sliding fit; a seal being interposed between the walls of the last-mentioned bore and -the capillary tube to prevent loss of mercury but permitting the capillary tube to be extended and retracted manually to actas a piston to remove by a pumping action any air that may be entrapped in the aforesaid central bore and that may be entrapped in said rnicroporous tubing.

9. An electrochemical method of analysis character- 7 ized by the steps of causing the test solution under investigation to migrate through a porous wall on the other surface of which is a body of mercury, said porous wall being impermeable to the mercury; and applying an electrical potential to the mercury via a suitable electrode adapted to be in electrolytic contact with the test solution, so as to produce a desired electrode reaction between the test solution and the mercury.

10. The invention defined in claim 9, wherein the mercury is constantly flowing under low pressure.

11. The invention defined in claim 9, wherein the test solution is caused to flow constantly.

12. The invention defined in claim 9, wherein the mercury is constantly flowing under low pressure, and the test solution is caused to flow constantly.

13. An electrochemical method of analysis comprising putting a mass of mercury under slight pressure into contact with one face of a porous wall whose pores are small enough to preclude the mercury from entering them;

putting a test solution under investigation into contact with the other face of said porous wall, said pores permitting the test solution to migrate through said wall to contact the mercury; and putting a suitable electrode in electrolytic contact with the test solution; and applying an electrical potential to the mercury versus said electrode so as to produce a desired electrode reaction between the test solution and the mercury.

7 8 14. The invention defined in claim 13, wherein the References Cited in tho-file of this patent mercury is caused to fiow along said wall constantly. h w a T 15. The invention defined in claim 13, wherein the test UNITED STATLS PATbbTS solution is caused to flow constantly. 2 -7 871 k 1 4 16. The invention defined in claim 13, wherein the 5 :2 :5 ag 5 3 the mercury is constantly flowing under said pressure, and 2861926 Jacobs); Nov 1958 the test solution is caused to flow constantly. 

9. AN ELECTROCHEMICAL METHOD OF ANALYSIS CHARACTERIZED BY THE STEPS OF CAUSING THE TEST SOLUTION UNDER INVESTIGATION TO MIGRATE THROUGH A POROUS WALL ON THE OTHER SURFACE OF WHICH IS A BODY OF MERCURY; SAID POROUS WALL BEING IMPERMEABLE TO THE MERCURY; AND APPLYING AN ELECTRICAL POTENTIAL TO THE MERCURY VIA A SUITABLE ELECTRODE ADAPTED TO BE IN ELECTROLYTIC CONTACT WITH THE TEST SOLUTION, SO AS TO PRODUCE A DESIRED ELECTRODE REACTION BETWEEN THE TEST SOLUTION AND THE MERCURY. 